Added a stringContaining asymmetric equality tester

* Fixes #1923.

spec/core/asymmetric_equality/StringContainingSpec.js

@@ -0,0 +1,27 @@
+describe('StringContaining', function() {
+  it('searches for a provided substring when the expected is a String', function() {
+    var matcher = new jasmineUnderTest.StringContaining('foo');
+
+    expect(matcher.asymmetricMatch('barfoobaz')).toBe(true);
+    expect(matcher.asymmetricMatch('barbaz')).toBe(false);
+  });
+
+  it('raises an Error when the expected is not a String', function() {
+    expect(function() {
+      new jasmineUnderTest.StringContaining(/foo/);
+    }).toThrowError(/not a String/);
+  });
+
+  it('fails when the actual is not a String', function() {
+    var matcher = new jasmineUnderTest.StringContaining('x');
+    expect(matcher.asymmetricMatch(['x'])).toBe(false);
+  });
+
+  it("jasmineToString's itself", function() {
+    var matching = new jasmineUnderTest.StringContaining('foo');
+
+    expect(matching.jasmineToString()).toEqual(
+      '<jasmine.stringContaining("foo")>'
+    );
+  });
+});