Fix casing of renamed fields

Author: jaredpar

src/Microsoft.DotNet.CodeFormatting.Tests/Rules/PrivateFieldNamingRuleTests.cs

@@ -113,5 +113,36 @@ class C3
 
             Verify(text, expected, runFormatter: true);
         }
+
+        /// <summary>
+        /// If the name is pascal cased make it camel cased during the rewrite.  If it is not
+        /// pascal cased then do not change the casing.
+        /// </summary>
+        [Fact]
+        public void NameCasingField()
+        {
+            var text = @"
+class C
+{
+    int Field;
+    static int Other;
+    int GCField;
+    static int GCOther;
+}
+";
+
+            var expected = @"
+class C
+{
+    int _field;
+    static int s_other;
+    int _GCField;
+    static int s_GCOther;
+}
+";
+
+            Verify(text, expected, runFormatter: false);
+
+        }
     }
 }

src/Microsoft.DotNet.CodeFormatting/Rules/PrivateFieldNamingRule.cs

@@ -215,6 +215,11 @@ private static string GetNewFieldName(IFieldSymbol fieldSymbol)
                 return fieldSymbol.Name;
             }
 
+            if (name.Length > 2 && char.IsUpper(name[0]) && char.IsLower(name[1]))
+            {
+                name = char.ToLower(name[0]) + name.Substring(1);
+            }
+
             if (fieldSymbol.IsStatic)
             {
                 // Check for ThreadStatic private fields.