Added Cdata class to allow writing of CDATA values

[zyura]

Sometimes it's desired to use CDATA in the resulting XML file instead of regular XML escaping.

Cdata wrapper class allows to achieve this:

$html_desc = '<p>Some HTML code</p>';
$offer = (new OfferSimple())
    ->setDescription(new Cdata($html_desc))

will result in

<description><![CDATA[<p>Some HTML code</p>]]></description>

instead of

<description>&lt;p&gt;Some HTML code&lt;/p&gt;</description>

[Bukashk0zzz]

Hi, Thank you for PR.

There are some code style problems https://travis-ci.org/Bukashk0zzz/YmlGenerator/jobs/516475287

Can you please fix them before I merge this?

[zyura]

Sure, I'll commit the fixed version soon.

[codecov-io]

Codecov Report

Merging #21 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #21   +/-   ##
=======================================
  Coverage       100%   100%           
- Complexity      245    248    +3     
=======================================
  Files            17     18    +1     
  Lines           682    691    +9     
=======================================
+ Hits            682    691    +9

Impacted Files	Coverage Δ	Complexity Δ
src/Generator.php	100% <100%> (ø)	47 <0> (+1)	⬆️
src/Cdata.php	100% <100%> (ø)	2 <2> (?)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6ae4bd9...8429427. Read the comment docs.

[zyura]

Fixed all code style issues, added a test for the Cdata class: OfferCdataGeneratorTest.
Had to raise method visibility of AbstractGeneratorTest::createOffers() to protected because it didn't allow to define custom offer description: description set in overridden createOffer() was overwritten by

    ->setDescription($this->faker->sentence)