No max-width for autocomplete panel

Currently, the max-width is inherited from the mat-menu-base mixin forcing the panel to have  a width of 280px. Overriding it here with none so that the panel can expand out as necessary.

Fix based on comments in issue angular#3198.

src/lib/autocomplete/autocomplete.scss

@@ -15,7 +15,8 @@ $mat-autocomplete-panel-above-offset: -24px !default;
 .mat-autocomplete-panel {
   @include mat-menu-base();
   visibility: hidden;
-
+
+  max-width: none;
   max-height: $mat-autocomplete-panel-max-height;
   position: relative;
 
@@ -34,4 +35,4 @@ $mat-autocomplete-panel-above-offset: -24px !default;
   &.mat-autocomplete-hidden {
     visibility: hidden;
   }
-}
+}