[sol005robot] Set Headers for Accept ZIP/Plain text not implemented correctly

For tests which need to accept both application/zip and text/plain, the test is written as follows:

Set Headers    {"Accept": "${ACCEPT_PLAIN}"}
Set Headers    {"Accept": "${ACCEPT_ZIP}"}

In this case, the state change of the first call is lost, and only the second call has any effect. This means the test does not run as intended.