Handle server URL comparison correctly

[karol-maciaszek]

Chore summary IHttpOperation merger does not handle all the cases for comparison of server URLs.

Tasks

• [ ] host should be compared as case-insensitive
• [ ] path should be compared as case-sensitive
• [ ] variable names should be compared as case-sensitive (@philsturgeon is that correct assumption?)
• [ ] the case, when a variable is a part of hostname, should be handled correctly: host can be compared case-insensitive while variable name should be compared case-sensitively

Additional context This task is a followup for a discussion here: https://github.com/stoplightio/http-spec/pull/83#discussion_r389628980

[philsturgeon]

I don't have enough context to know if variable names should be case sensitive or not, I don't think it matters too much either way? Some people will expect it to work one way and others another.