Test the jaeger propagator fields

[MrAlias]

Test the functionality provided:

opentelemetry-go-contrib/propagators/jaeger/jaeger_propagator.go

Lines 153 to 156 in 836e94b

	// Fields returns the Jaeger header key whose value is set with Inject.
	func (jaeger Jaeger) Fields() []string {
	return []string{jaegerHeader}
	}

The test should verify the singular response of this method.

[hamza3202]

I am working on this.