Add case sensitive comparison option to elementContains and elementNotContains

[MurzNN]

Fixes https://github.com/minkphp/Mink/issues/829

[codecov[bot]]

Codecov Report

Merging #830 (30cc903) into master (19e5890) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #830   +/-   ##
=========================================
  Coverage     98.47%   98.47%           
- Complexity      345      347    +2     
=========================================
  Files            23       23           
  Lines           983      985    +2     
=========================================
+ Hits            968      970    +2     
  Misses           15       15           

Impacted Files	Coverage Δ
src/WebAssert.php	100.00% <100.00%> (ø)

---

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 19e5890...30cc903. Read the comment docs.