feat: use https by default

[ericdeansanchez]

To summarize the following, this PR changes the default behavior
of the UrlBuilder class. Before this change, the default behavior
used http by default. After this change, the default behavior uses
https by default.

When I began, I made a small change in the constructor definition and
ran the test. All passed (not intended, not expected). I removed the
change and began this PR by writing a failing test first. I re-ran the
test suite and the test,testExamplePlainUsesHttpsByDefault, failed
as intended. I then made the appropriate change in the constructor
definition, re-ran the tests, all passed.

• first, I wrote a failing test; it tests that this SDK does use
  https by default, it failed
• then the default constructor's default-parameter was changed to
  use $useHttps = true (instead of $useHttps = false)
• this change is to ensure that this SDK uses https by default
• with the new test testExamplePlainUsesHttpsByDefault, any
  subsequent changes to this behavior will be a "breaking change"
• the naming of testExamplePlainUsesHttpsByDefault was used to
  emulate the naming-style present in the test suite
• the structure of testExamplePlainUsesHttpsByDefault has been
  written to emulate the structure of similar tests (e.g.
  testExamplePlain)
• note: with the given API, this change cannot be tested in
  isolation and requires a call to createUrl
• note: the current API does not allow for incremental testing
  of individual builder-fields, e.g. there are no "getters" and the
  builder-fields are private:

private $domain;
private $useHttps;
private $signKey;

• the above note is looking forward to future tweaks to our API to
  ensure more complete test coverage.

Ensure at least one test case fails before change:
Note that the expected value is https.

Time: 95 ms, Memory: 10.00 MB

There was 1 failure:

1) UrlBuilderTest::testExamplePlainUsesHttpsByDefault
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'https://demos.imgix.net/bridge.png?h=100&w=100'
+'http://demos.imgix.net/bridge.png?h=100&w=100'

/Users/Eric/GitHub/imgix-php/tests/Imgix/Tests/UrlBuilderTest.php:33

FAILURES!
Tests: 50, Assertions: 354, Failures: 1.

Ensure all tests pass after change:
Both the expected and actual values concur.

PHPUnit 9.0.1 by Sebastian Bergmann and contributors.

..................................................                50 / 50 (100%)

Time: 93 ms, Memory: 10.00 MB

OK (50 tests, 354 assertions)

This single test should be sufficient for our purposes, here and now. It
is the only point of entry (i.e. no "getters"). UrlHelper does have
a getUrl method, but that is not near this change (getUrl is not
called until UrlBuilder.createUrl returns).

[sherwinski]

All the code changes lgtm 👍

Can we also modify the documentation to reflect these changes before merging?

[ericdeansanchez]

@sherwinski Yeah, we can. Let's keep this commit/PR to just the above. I'll open another PR to update the documentation after this has been merged.