JSON: encode NaNs and infinite floats as `null`

Such values are not allowed by the JSON spec. We match the behavior of
JSON.stringify in Firefox and Chrome.

fixes https://github.com/sandstorm-io/capnproto/issues/261

c++/src/capnp/compat/json-test.c++

@@ -127,8 +127,8 @@ const char ALL_TYPES_JSON[] =
     "  \"uInt16List\": [33333, 44444],\n"
     "  \"uInt32List\": [3333333333],\n"
     "  \"uInt64List\": [\"11111111111111111111\"],\n"
-    "  \"float32List\": [5555.5, inf, -inf, nan],\n"
-    "  \"float64List\": [7777.75, inf, -inf, nan],\n"
+    "  \"float32List\": [5555.5, null, null, null],\n"
+    "  \"float64List\": [7777.75, null, null, null],\n"
     "  \"textList\": [\"plugh\", \"xyzzy\", \"thud\"],\n"
     "  \"dataList\": [[111, 111, 112, 115], [101, 120, 104, 97, 117, 115, 116, 101, 100], [114, 102, 99, 51, 48, 57, 50]],\n"
     "  \"structList\": [\n"

c++/src/capnp/compat/json.c++

@@ -20,6 +20,7 @@
 // THE SOFTWARE.
 
 #include "json.h"
+#include <cmath>
 #include <unordered_map>
 #include <kj/debug.h>
 
@@ -237,9 +238,20 @@ void JsonCodec::encode(DynamicValue::Reader input, Type type, JsonValue::Builder
     case schema::Type::UINT8:
     case schema::Type::UINT16:
     case schema::Type::UINT32:
+      output.setNumber(input.as<double>());
+      break;
     case schema::Type::FLOAT32:
     case schema::Type::FLOAT64:
-      output.setNumber(input.as<double>());
+      {
+        double value = input.as<double>();
+        if (std::isinf(value) || std::isnan(value)) {
+          // These values are not allowed in the JSON spec. Setting the field as null matches the
+          // behavior of JSON.stringify in Firefox and Chrome.
+          output.setNull();
+        } else {
+          output.setNumber(value);
+        }
+      }
       break;
     case schema::Type::INT64:
       output.setString(kj::str(input.as<int64_t>()));

c++/src/capnp/compat/json.h

@@ -48,6 +48,8 @@ class JsonCodec {
   // - When encoding, all primitive fields are always encoded, even if default-valued. Pointer
   //   fields are only encoded if they are non-null.
   // - 64-bit integers are encoded as strings, since JSON "numbers" are double-precision floating
+  // - NaNs and infinite floating point numbers are not allowed by the JSON spec, and so are encoded
+  //   as null. This matches the behavior of `JSON.stringify` in at least Firefox and Chrome.
   //   points which cannot store a 64-bit integer without losing data.
   // - Data is encoded as an array of numbers in the range [0,255]. You probably want to register
   //   a handler that does something better, like maybe base64 encoding, but there are a zillion