fix(MdField): prevent MdInput and MdTextarea from emit input events directly (#1285)

* (Fix #1261) Don't emit input directly in MdInput and MdTextarea

* use spread operator (+babel) instead of Object.assign for browser compatiblity

Author: korylprince

src/components/MdField/MdInput/MdInput.vue

@@ -35,10 +35,9 @@
         return this.type === 'password'
       },
       listeners () {
-        return {
-          ...this.$listeners,
-          input: event => this.$emit('input', event.target.value)
-        }
+        var l = {...this.$listeners}
+        delete l.input
+        return l
       }
     },
     watch: {

src/components/MdField/MdTextarea/MdTextarea.vue

@@ -48,10 +48,9 @@
     },
     computed: {
       listeners () {
-        return {
-          ...this.$listeners,
-          input: event => this.$emit('input', event.target.value)
-        }
+        var l =  {...this.$listeners}
+        delete l.input
+        return l
       },
       textareaStyles () {
         return {