Fix warning: assigned but unused variable

There are several places where varaibles are assigned but not used.
The variables can be replaced with _, or prepended with _ to avoid a warning.
In one case the variable was removed because it was at the end of the method.

Addresses the following warnings:

lib/google/api_client.rb:493: warning: assigned but unused variable - key
lib/google/api_client/batch.rb:168: warning: assigned but unused variable - callback
lib/google/api_client/batch.rb:227: warning: assigned but unused variable - base
lib/google/api_client/batch.rb:271: warning: assigned but unused variable - protocol
lib/google/api_client/batch.rb:271: warning: assigned but unused variable - reason
lib/google/api_client/request.rb:247: warning: assigned but unused variable - request_env

lib/google/api_client.rb

@@ -490,7 +490,7 @@ module Google
       else
         check_cached_certs = lambda do
           valid = false
-          for key, cert in @certificates
+          for _key, cert in @certificates
             begin
               self.authorization.decoded_id_token(cert.public_key)
               valid = true

lib/google/api_client/batch.rb

@@ -165,7 +165,7 @@ module Google
         if @calls.nil? || @calls.empty?
           raise BatchError, 'Cannot make an empty batch request'
         end
-        parts = @calls.map {|(call_id, call, callback)| serialize_call(call_id, call)}
+        parts = @calls.map {|(call_id, call, _callback)| serialize_call(call_id, call)}
         build_multipart(parts, 'multipart/mixed', BATCH_BOUNDARY)
         super
       end
@@ -224,7 +224,7 @@ module Google
           raise BatchError, 'Invalid value for Content-ID: "%s"' % header
         end
 
-        base, call_id = header[1...-1].split('+')
+        _base, call_id = header[1...-1].split('+')
         return Addressable::URI.unencode(call_id)
       end
 
@@ -268,7 +268,7 @@ module Google
       def deserialize_call_response(call_response)
         outer_headers, outer_body = split_headers_and_body(call_response)
         status_line, payload = outer_body.split("\n", 2)
-        protocol, status, reason = status_line.split(' ', 3)
+        _protocol, status, _reason = status_line.split(' ', 3)
 
         headers, body = split_headers_and_body(payload)
         content_id = find_header('Content-ID', outer_headers)

lib/google/api_client/request.rb

@@ -244,7 +244,7 @@ module Google
           )
         end
 
-        request_env = http_request.to_env(connection)
+        http_request.to_env(connection)
       end
 
       ##