Fixes copyjson casting bug. (#1114)

VERSION_INFO

@@ -1 +1 @@
-1.5.0
+1.5.1

src/libawkward/io/json.cpp

@@ -47,7 +47,7 @@ namespace awkward {
       writer.Int64(value.GetInt());
     }
     else if (value.IsDouble()) {
-      writer.Int64((int64_t)value.GetDouble());
+      writer.Double(value.GetDouble());
     }
     else if (value.IsString()) {
       writer.String(value.GetString());

tests/test_1114-fix-copyjson-casting-bug.py

@@ -0,0 +1,30 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward-1.0/blob/main/LICENSE
+
+from __future__ import absolute_import
+
+import json
+
+import pytest  # noqa: F401
+import numpy as np  # noqa: F401
+import awkward as ak  # noqa: F401
+
+
+def test():
+    f = ak.forms.Form.fromjson(
+        json.dumps(
+            {
+                "class": "NumpyArray",
+                "itemsize": 8,
+                "format": "d",
+                "primitive": "float64",
+                "parameters": {"thing": 1.23},
+            }
+        )
+    )
+    assert json.loads(repr(f)) == {
+        "class": "NumpyArray",
+        "itemsize": 8,
+        "format": "d",
+        "primitive": "float64",
+        "parameters": {"thing": 1.23},  # not 1 (int)
+    }