Oauth1 signing: fix error message on invalid Content-Type header
Oauth1 signing: fix error message on invalid Content-Type header

Content-Type should be application/x-www-form-urlencoded when the
request to sign has a body. If it's not the case a ValueError was raised
saying Content-Type was missing, while it actually could just be wrongly
set (e.g. to application/json). Fix the error message to reflect the
real problem: expected & given Content-Type.

bjmc/oauthlib,idan/oauthlib,mick88/oauthlib,barseghyanartur/oauthlib,hirokiky/oauthlib,cyrilchaponeverysens/oauthlib,oauthlib/oauthlib,Blitzen/oauthlib,singingwolfboy/oauthlib,skion/oauthlib-oidc,metatoaster/oauthlib,armersong/oauthlib,garciasolero/oauthlib