HttpClient should support default headers

[dcsobral]

While looking at github's api docs I noticed that UserAgent is mandatory, and quickly verified github4s doesn't add it by default. While http4s has its own default, I decided I wanted to use "github4s" instead. However, I quickly realized that meant adding a header to every github4s call, which is very redundant.

For cases like this and, for that matter, github feature flags, it would be very convenient if HttpClient would take default headers.

[BenFradet]

created #461 👍