Fix handling of types with custom formatters that are convertible to std::string_view

[denizevrenci]

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

I commented at the PR (#1342) as well but types that are convertible to std::string_view have ambiguous lookup after that change.

I am not sure if this is the right solution but I wanted to start the discussion here. And this change fixes the ambiguous lookup in my case.

[vitaut]

Thanks! Could you add a test case similar to

fmt/test/core-test.cc

Lines 620 to 627 in 123e7f7

	struct explicitly_convertible_to_std_string_view {
	explicit operator std::string_view() const { return "foo"; }
	};
	TEST(FormatterTest, FormatExplicitlyConvertibleToStdStringView) {
	EXPECT_EQ("foo",
	fmt::format("{}", explicitly_convertible_to_std_string_view()));
	}

?

[denizevrenci]

I dug a little deeper into this issue and understood that the conflict is caused by the existence of a custom formatter. My earlier fix disables the custom formatter and uses the default std::string_view formatter. So the test that I just added does not pass with or without the fix.

Here is the error: https://travis-ci.org/fmtlib/fmt/jobs/620950783#L716

[denizevrenci]

I pushed a new fix that I think addresses this issue.

[vitaut]

LGTM, thanks!