[FIX] confusion between precision and min_precision

`precision` expresses precision within the provided value and won't expand it if the max expression of the value is smaller
`min_precision` will expand the value if the max expression of the value is smaller

Author: geertweening

src/js/ripple/amount.js

@@ -1120,11 +1120,12 @@ Amount.prototype.to_human = function(opts) {
         }
         fraction_part = '';
       } else {
-        fraction_part = Math.round(fraction_part / Math.pow(10, fraction_part.length - opts.precision)).toString();
+        var precision = Math.min(opts.precision, fraction_part.length);
+        fraction_part = Math.round(fraction_part / Math.pow(10, fraction_part.length - precision)).toString();
 
         // because the division above will cut off the leading 0's we have to add them back again
         // XXX look for a more elegant alternative
-        while (fraction_part.length < opts.precision) {
+        while (fraction_part.length < precision) {
           fraction_part = '0' + fraction_part;
         }
       }

test/amount-test.js

@@ -69,7 +69,13 @@ describe('Amount', function() {
       assert.strictEqual(Amount.from_human("0.00012345 XAU").to_human({precision:6}), '0.000123');
     });
     it('to human, precision 16', function() {
-      assert.strictEqual(Amount.from_human("0.00012345 XAU").to_human({precision:16}), '0.0001234500000000');
+      assert.strictEqual(Amount.from_human("0.00012345 XAU").to_human({precision:16}), '0.00012345');
+    });
+    it('to human, precision 16, min_precision 16', function() {
+      assert.strictEqual(Amount.from_human("0.00012345 XAU").to_human({precision:16, min_precision:16}), '0.0001234500000000');
+    });
+    it('to human, precision 16, min_precision 12', function() {
+      assert.strictEqual(Amount.from_human("0.00012345 XAU").to_human({precision:16, min_precision:12}), '0.000123450000');
     });
     it('to human, precision 0, first decimal 4', function() {
       assert.strictEqual(Amount.from_human("0.4 XAU").to_human({precision:0}), '0');
@@ -80,6 +86,21 @@ describe('Amount', function() {
     it('to human, precision 0, first decimal 8', function() {
       assert.strictEqual(Amount.from_human("0.8 XAU").to_human({precision:0}), '1');
     });
+    it('to human, precision 0, precision 16', function() {
+      assert.strictEqual(Amount.from_human("0.0 XAU").to_human({precision:16}), '0.0');
+    });
+    it('to human, precision 0, precision 8, min_precision 16', function() {
+      assert.strictEqual(Amount.from_human("0.0 XAU").to_human({precision:8, min_precision:16}), '0.0000000000000000');
+    });
+    it('to human, precision 0, first decimal 8', function() {
+      assert.strictEqual(Amount.from_human("0.8 XAU").to_human({precision:0}), '1');
+    });
+    it('to human, precision 6, min_precision 6, max_sig_digits 20', function() {
+      assert.strictEqual(Amount.from_human("0.0 XAU").to_human({precision: 6, min_precision: 6, max_sig_digits: 20}), '0.000000');
+    });
+    it('to human, precision 16, min_precision 6, max_sig_digits 20', function() {
+      assert.strictEqual(Amount.from_human("0.0 XAU").to_human({precision: 16, min_precision: 6, max_sig_digits: 20}), '0.000000');
+    });
   });
   describe('from_human', function() {
     it('1 XRP', function() {