Initialize Net::HTTP::Post with URI instead of string

[marcboquet]

Initializing it with a string causes path to be incorrect:

url = URI.parse('https://www.patreon.com/api/oauth2/token')
req = Net::HTTP::Post.new(url.to_s)
req.path
# => "https://www.patreon.com/api/oauth2/token"

As opposed to

url = URI.parse('https://www.patreon.com/api/oauth2/token')
req = Net::HTTP::Post.new(url)
req.path
# => "/api/oauth2/token"

ruby-doc for Net::HTTP::Post shows initialization using a URI too.

Initializing with a string causes apm-agent-ruby to raise with URI::InvalidURIError. Related issue: elastic/apm-agent-ruby#834