Improve error message when server returns invalid data

[ivoanjo]

As discussed in #121, the error message you get when a web server returns non-json hal data or the Content-Type is incorrect is rather unclear on what happened.

This commit adds a check when creating a Resource that the received representation is something that makes sense.

This turns the error message for an example such as Hyperclient.new('http://www.google.com/').foo from

/lib/hyperclient/collection.rb:78:in `method_missing': undefined method
`fetch' for #<String:0x005654b3898628> (NoMethodError)
        from /lib/hyperclient/resource.rb:87:in `block in method_missing'
        from /lib/hyperclient/resource.rb:85:in `each'
        from /lib/hyperclient/resource.rb:85:in `method_missing'
        from /lib/hyperclient/link.rb:129:in `method_missing'
        from example.rb:3:in `<main>'

to

/lib/hyperclient/resource.rb:90:in `validate': Invalid representation
for resource (got String, expected Hash). Is your web server returning
JSON HAL data with a 'Content-Type: application/hal+json' header?
(Hyperclient::InvalidRepresentationError)
        from /lib/hyperclient/resource.rb:43:in `initialize'
        from /lib/hyperclient/link.rb:179:in `new'
        from /lib/hyperclient/link.rb:179:in `http_method'
        from /lib/hyperclient/link.rb:89:in `_get'
        from /lib/hyperclient/link.rb:84:in `_resource'
        from /lib/hyperclient/link.rb:128:in `method_missing'
        from example.rb:3:in `<main>'

e.g. we now validate immediately after the request, and provide meaningful information and even a hint towards the possibly-missing header.

[dblock]

Great job, merging.