fix(select): rollback number parsing on value attr

closes #4615

As per the discussion in #4615, please use `ng-value` if you need
a non-string value from an md-option for the `ng-model` of a select.

src/components/select/select.js

@@ -673,7 +673,7 @@ function OptionDirective($mdButtonInkRipple, $mdUtil) {
     if (angular.isDefined(attr.ngValue)) {
       scope.$watch(attr.ngValue, setOptionValue);
     } else if (angular.isDefined(attr.value)) {
-      setOptionValue(isNaN(attr.value) ? attr.value : Number(attr.value));
+      setOptionValue(attr.value);
     } else {
       scope.$watch(function() {
         return element.text();

src/components/select/select.spec.js

@@ -91,15 +91,6 @@ describe('<md-select>', function() {
     //expect($document[0].activeElement).toBe(select[0]);
   }));
 
-  it('should not convert numbers to strings', inject(function($compile, $rootScope) {
-    $rootScope.value = 1;
-    $compile('<md-select ng-model="value">' +
-      '<md-option ng-repeat="value in [1, 2, 3]" value="{{value}}">{{value}}</md-option>' +
-      '</md-select>')($rootScope);
-    $rootScope.$digest();
-    expect($rootScope.value).toBe(1);
-  }));
-
   describe('input container', function() {
     it('should set has-value class on container for non-ng-model input', inject(function($rootScope, $document) {
       var el = setupSelect('ng-model="$root.model"', [1, 2, 3]);