URL::build(): Accept host-only URLs

[behnam]

Accept host-only URLs, which are buildable using the parsing method as
`//host-name'.

Also, add check for setting port without a host, along-side test case
for it.

[behnam]

This could be considered an API-breaking change, since a call previously raising error now doesn't do so, and some application logic may depend on it. What do you think?

Btw, what's the best way to track API-breaking changes here?

[codecov]

Codecov Report

Merging #217 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #217   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files           2        2           
  Lines         582      582           
  Branches      140      140           
=======================================
  Hits          580      580           
  Partials        2        2

Impacted Files	Coverage Δ
yarl/__init__.py	100% <100%> (ø)	⬆️

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[behnam]

#ping Would you like me to make any changes here?

[webknjaz]

LGTM, but I'll leave it up to @asvetlov to consider consequences.

Regarding tracking changes it's probably just that the project follows semver. I'd also add entry to CHANGES.rst.

[asvetlov]

Thanks!