Make requestBuilder a pure function

There's no need for IO, so make the function which runs a
RequestBuilder action a pure one.

This is a breaking change, unfortunately, but addresses the driving
concern in #9 and #48 and also afcowie/http-common#9. There's a branch on [http-common](https://github.com/afcowie/http-common) which implements this, obviously.

Good idea? Probably long overdue.

AfC