Implement Errors#from_array in terms of #from_hash

[seanpdoyle]

This commit changes the ActiveResource::Errors#from_array method to be implemented in terms of its complementary #from_hash method.

Rather than modifying the Errors instance itself (like managing attribute and :base assignment, clearing, etc), #from_array will instead transform the Array instance into a Hash keyed by known attributes. Once the Hash is constructed, the method hands it off to the #from_hash method.

The goal of this change is to reduce the duplicated burden of responsibility.

Along with the implementation change, this commit also includes explicit coverage of the Errors#from_xml and Errors#from_json methods. They're part of the gem's public API (since they're documented and not marked # :nodoc:), but aren't explicitly tested elsewhere in the suite.

There are two XML-focused tests. The first covers the case where the payload has multiple <error> elements:

Hash.from_xml "<errors><error>Name can't be blank</error><error>Email can't be blank</error></errors>"
 # => {"errors"=>{"error"=>["Name can't be blank", "Email can't be blank"]}}

The call to Hash.from_xml transforms each <error> element into an item in the Array.

The second test covers the case where the payload has a single payload has a single <error> element:

Hash.from_xml "<errors><error>Name can't be blank</error></errors>"
 # => {"errors"=>{"error"=>"Name can't be blank"}}

In this case, The call to Hash.from_xml transforms the <error> element into a single String value.

Since the Errors#from_xml method is part of the public API, and since its implementation invokes Errors#from_array, dedicating explicit test coverage for both scenarios can help prevent regressions or breaks in backwards-compatibility.