String type must be strict.

It makes sense to coerce from more general -> more specific data types.
eg. string -> int, etc. But coercing the other way is problematic
in the case of chaining because the more general type will swallow the
specific type. eg. Setting `false` on type: [‘string’, ‘boolean’] will 
coerce the boolean to a string, and will never allow the value to be
a boolean.

src/config.coffee

@@ -667,12 +667,10 @@ Config.addSchemaEnforcers
           throw new Error("Validation failed at #{keyPath}, #{JSON.stringify(value)} must be a boolean or the string 'true' or 'false'")
 
   'string':
-    coerce: (keyPath, value, schema) ->
-      switch typeof value
-        when 'number', 'string'
-          value.toString()
-        else
-          throw new Error("Validation failed at #{keyPath}, #{JSON.stringify(value)} must be a string or number")
+    validate: (keyPath, value, schema) ->
+      unless typeof value is 'string'
+        throw new Error("Validation failed at #{keyPath}, #{JSON.stringify(value)} must be a string")
+      value
 
   'null':
     # null sort of isnt supported. It will just unset in this case