Improved error message for one assert function

[ochaplashkin]

What's the problem?

If the user makes a mistake in the assert_error_msg_content_equals:

t.assert_error_msg_content_equals("bad", error, "foo:1: bar")

where:

    "bad" - excepted error message
    error - built-in function (there may be another function here)
    "foo:1: bar" - argument for the function

then the following error will be returned:

Error message expected: "bad"
Error message received: "foo:1: bar"

The user thinks that a comparison is being made between foo:1: bar and bad. He notices this error and at the same time can write the following:

t.assert_error_msg_content_equals("foo:1: bar", error, "foo:1: bar")

Now he will get the following error:

Error message expected: "foo:1: bar"
Error message received: "foo:1: bar"

It seems that the function doesn't work cause these strings are equal. In fact, a comparison will be performed between foo:1: bar (excepted) and bar strings (because foo:1: will be dropped from the error message by regex).

The next use of the function will be correct:

t.assert_error_msg_content_equals("bar", error, "foo:1: bar")

What's the solution?

It is necessary to save the result of conversion after the gsub. So this way the user will see the actual strings that are being compared:

t.assert_error_msg_content_equals("bad", error, "foo:1: bar")
Error message expected: "bad"
Error message received: "bar"

Close #316