Remove charset from the response content-type

This is not required as JSON use UTF-8 by default (see RFC4627).
Make it homogeneous with error responses.
Prevent alert when /dev/urandom is not accessible

>> Warning: file_exists() [<a href='function.file-exists'>function.file-exists</a>]: open_basedir restriction in effect. File(/dev/urandom) is not within the allowed path(s): (/usr/lib/php:/usr/local/lib/php:/tmp) in /.../vendor/friendsofsymfony/oauth2-php/lib/OAuth2/OAuth2.php line 1190
Merge pull request #28 from felds/patch-1

Prevent alert when /dev/urandom is not accessible
Merge pull request #24 from GromNaN/patch-1

Remove charset from the response content-type