Issue Description:

For example in a code like this:

protocol SomeProtocol {
    func someFunction()
    func someFunction(parameter: String)
}

class SomeClass: SomeProtocol {
    func someFunction(parameter: String = "") {
        if parameter.characters.count > 0 {
            print("string: " + parameter)
        } else {
            print("empty string")
        }
    }
}

Compiler reports an error, that there's no implementation for func someFunction() in SomeClass. For example I got such report in playground:

Playground execution failed: error: MyPlayground.playground:38:7: error: type 'SomeClass' does not conform to protocol 'SomeProtocol'
class SomeClass: SomeProtocol {
      ^

MyPlayground.playground:34:10: note: protocol requires function 'someFunction()' with type '() -> ()'; do you want to add a stub?
    func someFunction()
         ^

MyPlayground.playground:39:10: note: candidate has non-matching type '(String) -> ()'
    func someFunction(parameter: String = "") {
         ^

but it's possible to execute following code:

let item = SomeClass()
item.someFunction()

So I believe this class could be treated as conforming to the SomeProtocol protocol.
The workaround is easy but requires boilerplate code. It would be great if compiler would take default argument values into account while checking protocol conformance.