Avoid deoptimization in generateCSS() (#204)

I was profiling the css() function and Chrome raised a flag on this
function:

> Not optimized: Bad value context for arguments value

More info on this warning:
GoogleChrome/devtools-docs#53 (comment)

Looking at the warning and the compiled version of this code, it seems
to do some things with `arguments` when using the default values
here, which is causing this deoptimization.

```js
var selectorHandlers = arguments.length <= 2 || arguments[2] === undefined ? [] : arguments[2];
var stringHandlers = arguments.length <= 3 || arguments[3] === undefined ? {} : arguments[3];
var useImportant = arguments.length <= 4 || arguments[4] === undefined ? true : arguments[4];
```

By removing the default values for the arguments, the deoptimization
disappears. I thought about adding logic that would provide values for
these arguments if they aren't defined, but since the only thing that
relies on that is tests I decided to just update the tests to always
pass all of the arguments.

In my benchmark, this does not seem to make much of a difference but it
still seems like a good idea to avoid things that the browser tells us
is deoptimized.

src/generate.js

@@ -139,9 +139,9 @@ export const defaultSelectorHandlers = [
 export const generateCSS = (
     selector /* : string */,
     styleTypes /* : SheetDefinition[] */,
-    selectorHandlers /* : SelectorHandler[] */ = [],
-    stringHandlers /* : StringHandlers */ = {},
-    useImportant /* : boolean */ = true
+    selectorHandlers /* : SelectorHandler[] */,
+    stringHandlers /* : StringHandlers */,
+    useImportant /* : boolean */
 ) /* : string */ => {
     const merged = styleTypes.reduce(recursiveMerge);
 

tests/generate_test.js

@@ -81,8 +81,8 @@ describe('generateCSSRuleset', () => {
     });
 });
 describe('generateCSS', () => {
-    const assertCSS = (className, styleTypes, expected, selectorHandlers,
-                       stringHandlers, useImportant) => {
+    const assertCSS = (className, styleTypes, expected, selectorHandlers = [],
+                       stringHandlers = {}, useImportant = true) => {
         const actual = generateCSS(className, styleTypes, selectorHandlers,
                                    stringHandlers, useImportant);
         assert.equal(actual, expected.split('\n').map(x => x.trim()).join(''));