Fixed bug #64874 ("json_decode handles whitespace and case-sensitivity incorrectly")

This fixes the JSON parser's handling of top-level primitives. Specifically, it fixes whitespace (previously `"[true] "` decoded fine but `" true"` didn't), and case (previously `"tRue"` decoded fine and `[tRue]` did not, however both are RFC non-compliant). This is a tiny change, but because of the very unlikely case that there was PHP code relying on deserialising strings like `"Null"`, despite RFC non-compliance and no JSON serialisers outputting it, I must classify this as "backwards-incompatible". Hence, this should be merged into master and hopefully PHP 5.6.

~~I have a separate request which is the backwards-compatible half of this request that only fixes the whitespace issue. It is supposed to be merged into PHP 5.4 and PHP 5.5, since it is a non-backwards-incompatible bug fix: https://github.com/php/php-src/pull/456~~

**EDIT:** The backwards-compatible portion is now into 5.4, 5.5 and master. Hence this request now purely concerns the case-sensitivity portion.