baggage: Fix escaping in Member.String

[pellared]

Why

%20 (whitespace) is improperly encoded as +.

Test TestMemberString result before the fix:

--- FAIL: TestMemberString (0.00s)
    /home/rpajak/repos/opentelemetry-go/baggage/baggage_test.go:878: 
        	Error Trace:	/home/rpajak/repos/opentelemetry-go/baggage/baggage_test.go:878
        	Error:      	Not equal: 
        	            	expected: "key=%3B+"
        	            	actual  : "key=%3B%20"

I noticed the bug when working on #4755.

What

Only CTLs, whitespace, DQUOTE, comma, semicolon, backslash have to be escaped.

Characters like +, =, - do NOT need to be escaped - they are acceptable. Therefore, some tests are updated as they are no longer escaped.

Reference: https://www.w3.org/TR/baggage/#definition

Take notice that url.PathUnescape is used for decoding then it also makes more sense to use url.PathEscape to encode. Not mentioning that using url.QueryEscape simply produces invalid value.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e3bf787) 82.2% compared to head (bde4937) 82.2%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4756   +/-   ##
=====================================
  Coverage   82.2%   82.2%           
=====================================
  Files        226     226           
  Lines      18342   18344    +2     
=====================================
+ Hits       15085   15087    +2     
  Misses      2975    2975           
  Partials     282     282           

Files	Coverage Δ
baggage/baggage.go	100.0% <100.0%> (ø)

[pellared]

I created:

• baggage: Member.String should escape only the characters that MUST be escaped #4773

I am waiting until tomorrow before merging.