fixed a MMAssertMarkdownEqualsHTML issue
fixed a MMAssertMarkdownEqualsHTML issue

XCTAssertEqualObjects(nil, nil); could lead to all the test cases are
passed.

albertbori/MMMarkdown,gonefish/MMMarkdown,iosdev-republicofapps/MMMarkdown,mdiep/MMMarkdown,iosdev-republicofapps/MMMarkdown,albertbori/MMMarkdown,albertbori/MMMarkdown,mdiep/MMMarkdown,AgileBits/MMMarkdown,AgileBits/MMMarkdown,gonefish/MMMarkdown