That seems to work fine, so I thought I might as well add a test and document it in the README.

Symptom:
----------
Trying to create an object sending a seemingly correct JSON request in a
POST message - was giving a 404 not found error.  When looking at the cause
in the browser devtools - the body of the response contained "Not found (input)"

Cause:
-------
We were trying to validate parameters - even when the POST body
contained invalid JSON.  We call json_decode($data) in retrieveInputs
The JSON request was malformed and it had omitted the double quotes.
example: { fieldName : "fieldValue" }

Expected behavior and fix:
---------------------------
A POST request with malformed JSON should get a 400 "bad request"
response. Even more helpful would be an error code indicating why the
JSON was bad.

If we call json_decode(), afterwards, we immediately call
json_last_error(), and if the return value does not match
JSON_ERROR_NONE, we send it across bundled in a 400 response, so that
the user may rectify the cause by looking at the exact error message.

The possible error messages right now are:

    CODE  CONSTANT-ERROR MESSAGE

    0     JSON_ERROR_NONE-No error has occurred
    1     JSON_ERROR_DEPTH-The maximum stack depth has been exceeded
    2     JSON_ERROR_STATE_MISMATCH-Invalid or malformed JSON
    3     JSON_ERROR_CTRL_CHAR-Control character error, possibly incorrectly encoded
    4     JSON_ERROR_SYNTAX-Syntax error
    5     JSON_ERROR_UTF8-Malformed UTF-8 characters, possibly incorrectly encoded
    6     JSON_ERROR_RECURSION-One or more recursive references in the value to be encoded
    7     JSON_ERROR_INF_OR_NAN-One or more NAN or INF values in the value to be encoded
    8     JSON_ERROR_UNSUPPORTED_TYPE-A value of a type that cannot be encoded was given
    9     JSON_ERROR_INVALID_PROPERTY_NAME-A property name that cannot be encoded was given
    10    JSON_ERROR_UTF16-Malformed UTF-16 characters, possibly incorrectly encoded

After the fix:
-------------
On malformed JSON in the POST body - the following is returned:
    The request could not be understood by the server due to malformed
    syntax. The client SHOULD NOT repeat the request without modifications.
    (Error decoding input JSON. json_last_error code: 4)

If REQUEST_METHOD is not set - we throw an exception that says something
like:
    'Bad request (Error decoding input JSON. json_last_error code: 4)'

In order to accommodate this change in behavior - in the test suite, we
need a new method expectPattern (as the error code in the end of the
string may change, but the starting pattern stays the same - (/^Bad
request.*$/)