fix(functions): missing body when Content-Type header supplied by dev

[mandarini]

Moved from: supabase/functions-js#90
Author: @ggaabe

What kind of change does this PR introduce?

Fixes null body when user supplies Content-Type in headers of supabase.functions.invoke, ie: invoked like this

  const response = await supabase.functions.invoke("stripe-webhook", {
    method: "POST",
    headers: {
   // this causes the body to be null
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      action: "finalizeClinicSetup",
      stripeSessionId,
      userId,
    }),
  });

What is the current behavior?

The request body sent over the network will be null, instead of the supplied body value, if a Content-Type header is supplied

Please link any relevant issues here.

What is the new behavior?

If user supplies a Content-Type header, the body will contain its original value supplied by the developer

[coveralls]

coverage: 95.455% (+13.0%) from 82.5%
when pulling 0054692 on fix/functions-client-fix
into 188fa17 on master.