CAMEL-20624: Fix bug in camel-http to avoid duplicate OAuth2 Authorization headers

[joshuagrisham-karolinska]

Description

Uses HttpRequest setHeader instead of addHeader to ensure that duplicate of "Authorization" header will not be created when using camel-http's OAuth2 support. See: https://hc.apache.org/httpcomponents-core-5.1.x/current/httpcore5/apidocs/org/apache/hc/core5/http/HttpMessage.html#setHeader(org.apache.hc.core5.http.Header)

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