fix($http): config.param should expand array values properly

Today, calling e.g. $http(url, { params: { a: [1,2,3] } }) results in a query
string like "?a=%5B1%2C2%2C3%5D" which is undesirable. This commit enhances
buildURL to createa query string like "?a=1&a=2&a=3".

BREAKING CHANGE: if the server relied on the buggy behavior then either the
backend should be fixed or a simple serialization of the array should be done
on the client before calling the $http service.

Closes #1363