Correct the database mirroring partner envchange handle code.

[andrei-mihaila]

Got this error when trying to do a query against an MSSQL version 14 instance (or 2017)

** (MatchError) no match of right hand side value: <<15, 69, 0, 67, 0, <redacted> ...>>                                                                                                                                            
    (tds 2.1.1) lib/tds/tokens.ex:362: Tds.Tokens.decode_envchange/2                                                                                                                         
    (tds 2.1.1) lib/tds/tokens.ex:52: Tds.Tokens.decode_tokens/2                                                                                                                             
    (tds 2.1.1) lib/tds/messages.ex:66: Tds.Messages.parse/3                                                                                                                                 
    (tds 2.1.1) lib/tds/protocol.ex:507: Tds.Protocol.decode/2                                                                                                                               
    (tds 2.1.1) lib/tds/protocol.ex:562: Tds.Protocol.login/1                                                                                                                                
    (tds 2.1.1) lib/tds/protocol.ex:408: Tds.Protocol.connect/2                                                                                                                              
    (db_connection 2.2.2) lib/db_connection/connection.ex:69: DBConnection.Connection.connect/2                                                                                              
    (connection 1.0.4) lib/connection.ex:622: Connection.enter_connect/5                                                                                                                     
    (stdlib 3.13) proc_lib.erl:226: :proc_lib.init_p_do_apply/3                                                                                                                              
Last message: nil                                                                                                                                                                            
State: Tds.Protocol

This pull request fixes it for me (used the other case clauses and this TDS doc to come up with the fix)

Also, thank you for this lib!

[coveralls]

Coverage decreased (-0.07%) to 72.904% when pulling 46afb5f on andrei-mihaila:master into 608bce7 on livehelpnow:master.

[coveralls]

Coverage increased (+0.1%) to 73.129% when pulling 46afb5f on andrei-mihaila:master into 608bce7 on livehelpnow:master.

[mjaric]

Thanks, I will pass over other cases just in case and push this soon