ddtrace/tracer: fix a bug with the x-datadog-tags header parser

[Barbayar]

Context

After talked to the other integration teams, we updated x-datadog-tags format a little bit. Before, = wasn't allowed in the tag value. Now, = is allowed in the tag value. Which means, the following value is valid x-datadog-tags value.

tag1=value1,tag2=val=ue2===,tag3=value3

After the parser parsed the above value, we expect the following dictionary.

{
    "tag1": "value1",
    "tag2": "val=ue2===",
    "tag3": "value3"
}