Add Headers.get_as_optional_string and get_as_string

[muddyfish]

Description of change

Refactors Headers to have two new public methods: get_as_optional_string and get_as_string.

Refactor head_object and put_object to use new header methods rather than custom implementations

Relevant issues: N/A

Does this change impact existing behavior?

Changes log format slightly by making "Header string was not valid" text part of HeadersError.

Does this change need a changelog entry in any of the crates?

No

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[dannycjones]

LGTM!

[dannycjones]

nit for both:

Suggested change

	/// Get a single header by name as a string.
	pub fn get_as_string<H: AsRef<OsStr>>(&self, name: H) -> Result<String, HeadersError> {
	let header = self.get(name)?;
	let value = header.value();
	if let Some(s) = value.to_str() {
	Ok(s.to_string())
	} else {
	Err(HeadersError::Invalid(value.clone()))
	}
	}
	/// Get a single header by name as a [String].
	pub fn get_as_string<H: AsRef<OsStr>>(&self, name: H) -> Result<String, HeadersError> {
	let header = self.get(name)?;
	let value = header.value();
	if let Some(s) = value.to_str() {
	Ok(s.to_string())
	} else {
	Err(HeadersError::Invalid(value.clone()))
	}
	}

[dannycjones]

Ship it!