Add hasContentType matcher, HTTP status consts #37

Test Results ✅

Report	Passed	Time
strcalc/build/test-results/test-frontend-browser/TESTS-TestSuites.xml	1✅	4s
strcalc/build/test-results/test-frontend/TESTS-TestSuites.xml	1✅	1s
strcalc/build/test-results/test-large/TESTS-TestSuites.xml	1✅	3s
strcalc/build/test-results/test-medium-coverage/TESTS-TestSuites.xml	1✅	299ms
strcalc/build/test-results/test-medium/TESTS-TestSuites.xml	1✅	204ms
strcalc/build/test-results/test/TESTS-TestSuites.xml	1✅	16ms

1 tests were completed in 4s with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
main.test.js	1✅			635ms

✅ String Calculator UI > initial state > contains the "Hello, World!" placeholder

1 tests were completed in 1s with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
main.test.js	1✅			539ms

✅ String Calculator UI > initial state > contains the "Hello, World!" placeholder

1 tests were completed in 3s with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
WebApplicationTest	1✅			3s

com.mike_bland.training.testing.stringcalculator.WebApplicationTest
  ✅ testPlaceholder()

1 tests were completed in 299ms with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
ServletContractTest	1✅			299ms

com.mike_bland.training.testing.stringcalculator.ServletContractTest
  ✅ addEndpointPlaceholder()

1 tests were completed in 204ms with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
ServletContractTest	1✅			204ms

com.mike_bland.training.testing.stringcalculator.ServletContractTest
  ✅ servesCorrectLandingPage()

1 tests were completed in 16ms with 1 passed, 0 failed and 0 skipped.

Test suite	Passed	Failed	Skipped	Time
StringCalculatorTest	1✅			16ms

com.mike_bland.training.testing.stringcalculator.StringCalculatorTest
  ✅ testPlaceholder()