replace unnecessary usage of NSString with String

Source/Request.swift

@@ -518,7 +518,7 @@ extension Request: CustomDebugStringConvertible {
 
         if let
             HTTPBodyData = request.HTTPBody,
-            HTTPBody = NSString(data: HTTPBodyData, encoding: NSUTF8StringEncoding)
+            HTTPBody = String(data: HTTPBodyData, encoding: NSUTF8StringEncoding)
         {
             let escapedBody = HTTPBody.stringByReplacingOccurrencesOfString("\"", withString: "\\\"")
             components.append("-d \"\(escapedBody)\"")

Source/ResponseSerialization.swift

@@ -204,7 +204,7 @@ extension Request {
 
             let actualEncoding = encoding ?? NSISOLatin1StringEncoding
 
-            if let string = NSString(data: validData, encoding: actualEncoding) as? String {
+            if let string = String(data: validData, encoding: actualEncoding) {
                 return .Success(string)
             } else {
                 let failureReason = "String could not be serialized with encoding: \(actualEncoding)"

Tests/ParameterEncodingTests.swift

@@ -401,7 +401,7 @@ class URLParameterEncodingTestCase: ParameterEncodingTestCase {
 
         if let
             HTTPBody = URLRequest.HTTPBody,
-            decodedHTTPBody = NSString(data: HTTPBody, encoding: NSUTF8StringEncoding)
+            decodedHTTPBody = String(data: HTTPBody, encoding: NSUTF8StringEncoding)
         {
             XCTAssertEqual(decodedHTTPBody, "bar=2&foo=1", "HTTPBody is incorrect")
         } else {