Ensure space chars are correctly encoded for Android openFl targets

Android devices will receive a 400 (Bad Request) response for any URLs
containing spaces, but using StringTools.urlEncode() on the whole URL can
'over encode' it.

This fix simply replaces any space characters with the appropriate
encoding (%20).
Identify valid JSON strings and fix Android Content-type headers

Previously, if the data value being sent was already a JSON string, the
content-type was not being set correctly.

Additionally, Android devices don't seem to like having the Content-type
value being set directly in the headers from openFL. Instead we can set it
via flash.net.URLRequest::contentType
Merge pull request #13 from massiveinteractive/fix/android-url-encode

Ensure space chars are correctly encoded for Android openFl targets