replace relative protocol for discoverable endpoint

[slavone]

If some service provides discoverable oembed link with relative protocol ("//example.com/oembed"), the url will be invalid and the request will crash

Example:

<link rel="alternate" type="application/json+oembed" href="//www.playbuzz.com/api/oembed/?url=https://www.playbuzz.com/qzeyrr10/plan-a-vacation-and-well-tell-you-what-job-you-should-have&amp;format=json">

This can be fixed by merging the request url with discovered api endpoint. Example:

irb(main):004:0> req = URI.parse('http://example.com/resource/123')
=> #<URI::HTTP http://example.com/resource/123>
irb(main):005:0> endpoint = URI.parse('//example.com/api/oembed')
=> #<URI::Generic //example.com/api/oembed>
irb(main):006:0> req.merge endpoint
=> #<URI::HTTP http://example.com/api/oembed>

[coveralls]

Coverage remained the same at 95.215% when pulling 70825fa on slavone:fix/replace-relative-protocol into cd3f353 on ruby-oembed:master.

[coveralls]

Coverage remained the same at 95.215% when pulling 70825fa on slavone:fix/replace-relative-protocol into cd3f353 on ruby-oembed:master.

[coveralls]

Coverage increased (+0.003%) to 95.219% when pulling b7dd678 on slavone:fix/replace-relative-protocol into cd3f353 on ruby-oembed:master.

[coveralls]

Coverage increased (+0.003%) to 95.219% when pulling b7dd678 on slavone:fix/replace-relative-protocol into cd3f353 on ruby-oembed:master.

[coveralls]

Coverage increased (+0.003%) to 95.219% when pulling b7dd678 on slavone:fix/replace-relative-protocol into cd3f353 on ruby-oembed:master.