Add default headers to configuration

[rafaeljusto]

The dynamic values could be found in the HTTP headers (e.g. Etag). This new
configuration entry will work on the same way of the current defaults, but
being applied to the header instead of the JSON body.

Resolves #43

[rafaeljusto]

Travis failed on Go 1.9 due to some network issue, couldn't restart the build to see the green lights 😄

[sjkaliski]

@rafaeljusto thanks for the contribution! I'm wondering if there is any harm / incompatibility with just including this all in defaults?

[rafaeljusto]

Nop, no harm at all. Just I thought it was cleaner to keep headers and content defaults on different places. Unless there's a match for a header and JSON field such as:

GET /example HTTP/1.1
Host: i.want.to.be.overwrite

{
  "Host": "I don't want to be overwrite",
}

[rafaeljusto]

@sjkaliski Updated the PR to also get the HTTP headers from the defaults section.

[sjkaliski]

@rafaeljusto thanks! 🎉