override accept/content-type headers instead of adding. fixes #39

[HennerM]

The Content-Type and Accept request headers were appened, therefore the value wos concatenated, as reported in #39

[coveralls]

Coverage remained the same at 53.872% when pulling 8f6060b on HennerM:fix-duplicate-headers into dbbb34d on ghidoz:master.

[coveralls]

Coverage remained the same at 53.872% when pulling 3540202 on HennerM:fix-duplicate-headers into dbbb34d on ghidoz:master.

[HennerM]

I realised that the per-request headers are also appended on every following request and I think that this might not be the desired behaviour. Therefore I added a commit that fixes this and allows overriding the Accept and Content-Type header also.

[coveralls]

Coverage decreased (-0.5%) to 53.333% when pulling 5d38c52 on HennerM:fix-duplicate-headers into dbbb34d on ghidoz:master.

[ghidoz]

Perfect, thanks! 👍