Allow binary content downloads of assets

[acrobat]

For this to work I also had to change the way the default accept header, previously the header was appended which produced actually an incorrect accept header.

Previously is produced this (this was the same with preview headers being set):

Host: api.github.com
Accept: application/octet-stream
Accept: application/vnd.github.v3+json
User-Agent: php-github-api (http://github.com/KnpLabs/php-github-api)

With the default headers plugin being used

Host: api.github.com
Accept: application/octet-stream
User-Agent: php-github-api (http://github.com/KnpLabs/php-github-api)

Fixes #320

[GrahamCampbell]

missing phpdoc :)

[acrobat]

I didn't add it because it's fully documented in the typehint 😄