[3.2] Do not require trailing = for base64 encoded strings

[heifner]

All previous versions of nodeos required a trailing = for base64 encoded strings. fc::variant appends an extra = for all of its base64 encoded strings. However, non-fc base64 encoders do not add an extra = when one is not needed. This PR modifies fc::variant to support non-fc base64 encoded strings that do not have the extra =.

In 5.0, fc::variant was updated to not add the extra = to base64 encoded strings. See #1888. This PR allows nodeos to support the new 5.0 version of base64 encoded strings.

Issue #1461

[greg7mdp]

So I assume that if there is an extra = character at the end, base64_decode ignores it, right?

[heifner]

So I assume that if there is an extra = character at the end, base64_decode ignores it, right?

Yes. And there is an existing test that verifies that:

leap/libraries/libfc/test/test_base64.cpp

Lines 40 to 45 in bce1b0b

	BOOST_AUTO_TEST_CASE(base64dec_extraequals) try {
	auto input = "YWJjMTIzJCYoKSc/tPUB+n5h========="s;
	auto expected_output = "abc123$&()'?\xb4\xf5\x01\xfa~a"s;
	BOOST_CHECK_EQUAL(expected_output, base64_decode(input));
	} FC_LOG_AND_RETHROW();

[spoonincode]

fc::assert_exception?

[heifner]

The implementation uses .at() so std::out_of_range is maybe possible, although not sure if really is because of the while (pos < length_of_string)