T 072733: Update multipart form request serializer (#165)

- With the existing implementation we were having difficulties sending HTTP multipart requests to the RestAPI.
- The issue appeared to be the boundary value that is sent alongside these requests.
- The RestAPI endpoint in question (/Documents/LiabilityRelease?consumerId=[ID]) wasn't accepting the existing boundary value.
- In a multipart request, a boundary acts like a marker for each key/value pair in the request's form data.

Sources/Conduit/Networking/Serialization/MultipartFormRequestSerializer.swift

@@ -26,7 +26,19 @@ public final class MultipartFormRequestSerializer: HTTPRequestSerializer {
     static private let CRLF = "\r\n"
 
     private var formData: [FormPart] = []
-    private let contentBoundary = MultipartFormRequestSerializer.randomContentBoundary()
+
+    lazy var contentBoundary: String = {
+        let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
+        let lettersLength = UInt32(letters.count)
+
+        let randomCharacters = (0..<12).map { _ -> String in
+            let offset = Int(arc4random_uniform(lettersLength))
+            let characters = letters[letters.index(letters.startIndex, offsetBy: offset)]
+            return String(characters)
+        }
+
+        return randomCharacters.joined()
+    }()
 
     private lazy var inlineContentBoundary: String = {
         return "--\(contentBoundary)"
@@ -62,19 +74,6 @@ public final class MultipartFormRequestSerializer: HTTPRequestSerializer {
         return mutableRequest
     }
 
-    private static func randomContentBoundary() -> String {
-        let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
-        let lettersLength = UInt32(letters.count)
-
-        let randomCharacters = (0..<12).map { _ -> String in
-            let offset = Int(arc4random_uniform(lettersLength))
-            let characters = letters[letters.index(letters.startIndex, offsetBy: offset)]
-            return String(characters)
-        }
-
-        return "----------------------------\(randomCharacters.joined())"
-    }
-
     private func encodedDataFrom(string: String) -> Data? {
         return string.data(using: .utf8)
     }