Switch to default HTTP client instead of fasthttp

[xmonader]

Why? https://github.com/valyala/fasthttp#faq

Are there known net/http advantages comparing to fasthttp?

Yes:

net/http supports HTTP/2.0 starting from go1.6.
net/http API is stable, while fasthttp API constantly evolves.
net/http handles more HTTP corner cases.
net/http can stream both request and response bodies
net/http can handle bigger bodies as it doesn't read the whole body into memory
net/http should contain less bugs, since it is used and tested by much wider audience.

Originally posted by @xmonader in #4 (comment)

[sameh-farouk]

Update:
PR ready for review