making propagation keys case insensitive

[ashishagg]

HTTP RFC : https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2

HTTP header fields, which include general-header (section 4.5), request-header (section 5.3), 
response-header (section 6.2), and entity-header (section 7.1) fields, follow the same generic format 
as that given in Section 3.1 of RFC 822 [9]. Each header field consists of a name followed by a colon
(":") and the field value. *Field names are case-insensitive.*

When SpanContext is extracted using TextMapPropagator, keys in the carrier can be of different case. In many cases, carrier is HttpHeadersCarrier which exposes HTTP headers read from transport. According to RFC, these header names are case insensitive. Some of the servers and clients do not maintain case of the header provided/read.