parseJsonBody with readALLUTF8 instead of readAll + cast's

[wilzbach]

#1677

FWIW in theory JSON can be encoded in UTF-16 and UTF-32 (see references below), but the current implementation doesn't handle this as well, it just silently assumes UTF-8.

References

https://tools.ietf.org/html/rfc7159#section-8

JSON text SHALL be encoded in UTF-8, UTF-16, or UTF-32. The default
encoding is UTF-8, and JSON texts that are encoded in UTF-8 are
interoperable in the sense that they will be read successfully by the
maximum number of implementations; there are many implementations
that cannot successfully read texts in other encodings (such as
UTF-16 and UTF-32).

https://www.ietf.org/rfc/rfc4627.txt

JSON text SHALL be encoded in Unicode. The default encoding is
UTF-8.

Since the first two characters of a JSON text will always be ASCII
characters [RFC0020], it is possible to determine whether an octet
stream is UTF-8, UTF-16 (BE or LE), or UTF-32 (BE or LE) by looking
at the pattern of nulls in the first four octets.

       00 00 00 xx  UTF-32BE
       00 xx 00 xx  UTF-16BE
       xx 00 00 00  UTF-32LE
       xx 00 xx 00  UTF-16LE
       xx xx xx xx  UTF-8

[wilzbach]

(btw closely related: there's still #1677 in the queue - I guess you merge this after the 0.8.0 release next week?)

[s-ludwig]

Oops, I actually wanted to wait with this too until after 0.8.0... So then I'll just create a branch for the 0.8.0 release. That means that #1677 can be merged now, too.