Change response models 'Url' properties from Uri to string (#1585)

* Add convention test to ensure 'Url' properties are of type string

Closes #1582

* Change 'Url' properties from Uri to string

Global Find/Replace FTW!

* fix compilation errors in the integration tests project

* Extend 'Url' properties type check to request models

* Stick to convention tests naming convention

* Remove unused using directives in models

Changing from `Uri` to `string` means the `using System;`
directive was not needed anymore in some files

* Update exception message wording

* empty commit to trigger a new build - hopefully Travis passes

* add convention test to ensure request models have Uri 'Url' properties

* make request models 'Url' properties Uri

fix typo in convention test name

* revert some request models 'Url' properties as `string`

see https://github.com/octokit/octokit.net/pull/1585#issuecomment-297186728

* Change test so that all model types must have 'Url' properties of type string

 - Filter test input to only get types which have 'Url' properties
 - Merge response and request model types tests into one
 - Unparameterize the exception since we only check for the string type now

* Fix string.Format tokens

If this PR doesn't get rebased, it'll be my wall of shame FOREVER!

* and then it's even more embarrassing when the commit message says rebased but you really meant squashed

* Remove exclusion of `Release` from request models

Octokit/Models/Response/CommitContent.cs

@@ -1,5 +1,4 @@
-using System;
-using System.Diagnostics;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 
@@ -13,7 +12,7 @@ namespace Octokit
     {
         public RepositoryContentInfo() { }
 
-        public RepositoryContentInfo(string name, string path, string sha, int size, ContentType type, Uri downloadUrl, Uri url, Uri gitUrl, Uri htmlUrl)
+        public RepositoryContentInfo(string name, string path, string sha, int size, ContentType type, string downloadUrl, string url, string gitUrl, string htmlUrl)
         {
             Name = name;
             Path = path;
@@ -55,22 +54,22 @@ namespace Octokit
         /// <summary>
         /// URL to the raw content
         /// </summary>
-        public Uri DownloadUrl { get; protected set; }
+        public string DownloadUrl { get; protected set; }
 
         /// <summary>
         /// URL to this content
         /// </summary>
-        public Uri Url { get; protected set; }
+        public string Url { get; protected set; }
 
         /// <summary>
         /// The GIT URL to this content.
         /// </summary>
-        public Uri GitUrl { get; protected set; }
+        public string GitUrl { get; protected set; }
 
         /// <summary>
         /// The URL to view this content on GitHub.
         /// </summary>
-        public Uri HtmlUrl { get; protected set; }
+        public string HtmlUrl { get; protected set; }
 
         internal string DebuggerDisplay
         {