Cleaned JsonObject unit tests
Added a failng test
Cleaned JsonValue tests
Test casting a JsonValue to a string
Test casting a JsonValue to an int
Test casting a JsonValue to a long
Test casting a JsonValue to a bool
Test casting a JsonValue to a double
Test casting a JsonValue to a float
Test casting a JsonValue to a JsonArray
Reduced usages of EscapedString
Fixed casting JsonValue to string
Added static EscapedString::printTo
Made EscapedString pure static
Test JsonObject indexer with two values
Test getting a non existing key of a JsonObject
Removed test of undefined behavior
Changed behavior when JsonObject is full (now replace the last value).
Reduce code size by 64 bytes.
Removed unused JsonValue::null()