Merge pull request #172 from wkirschbaum/add-stringchars-protocol-check

Check exclude_protocol_implementation for String.Chars defimpl

lib/money/protocol/string_chars.ex

@@ -1,5 +1,7 @@
-defimpl String.Chars, for: Money do
-  def to_string(v) do
-    Money.to_string!(v)
+if !Money.exclude_protocol_implementation(String.Chars) do
+  defimpl String.Chars, for: Money do
+    def to_string(v) do
+      Money.to_string!(v)
+    end
   end
-end
+end