running irb from rvm does not use the prompt-mode option

rvm_irb_prompt_mode

UPDATE by peter_v 2010-12-19 15:53:00
* the issue also occurs with rvm use system
* so not related to rvm, but to irb
* discussed on irc with telemachus

peterv@ASUS:~$ rvm --version

rvm 1.1.6 by Wayne E. Seguin (wayneeseguin@gmail.com) [http://rvm.beginrescueend.com/]

peterv@ASUS:~$ rvm use
Now using system ruby.
peterv@ASUS:~$ ruby -v
ruby 1.8.7 (2010-01-10 patchlevel 249) [i486-linux]

peterv@ASUS:~$ irb  # with system ruby
irb(main):001:0> IRB.conf[:PROMPT_MODE]
=> :DEFAULT
irb(main):002:0> quit
peterv@ASUS:~$ irb --prompt-mode xmp
# this works as expected, PROMPT_I == nil
IRB.conf[:PROMPT_MODE]
    ==>:XMP
IRB.conf[:PROMPT][:XMP][:PROMPT_I]
    ==>nil
quit

peterv@ASUS:~$ rvm use 1.9.2  # now switching to one of the rvm supplied Ruby's
Using /home/peterv/.rvm/gems/ruby-1.9.2-p0
peterv@ASUS:~$ irb
ruby-1.9.2-p0 :001 > IRB.conf[:PROMPT_MODE]
 => :RVM 
ruby-1.9.2-p0 :002 > IRB.conf[:PROMPT][:RVM][:PROMPT_I]
 => "ruby-1.9.2-p0 :%03n > " 
ruby-1.9.2-p0 :003 > # this works as expected, RVM supplies its own prompt mode
ruby-1.9.2-p0 :004 >   quit

peterv@ASUS:~$ irb --prompt-mode xmp
ruby-1.9.2-p0 :001 > IRB.conf[:PROMPT_MODE]
 => :RVM 
ruby-1.9.2-p0 :002 > # this is not expected, irb under rvm, seems not
ruby-1.9.2-p0 :003 >   # to use the --prompt-mode option from the command line
ruby-1.9.2-p0 :004 >   quit

peterv@ASUS:~$ rvm use 1.8.7
Using /home/peterv/.rvm/gems/ruby-1.8.7-p302
peterv@ASUS:~$ irb --prompt-mode xmp
ruby-1.8.7-p302 :001 > IRB.conf[:PROMPT_MODE]
 => :RVM  # same behavior on ruby 1.8.7
ruby-1.8.7-p302 :002 > quit

UPDATE by peter_v : the issue also occurs with rvm use system

I now installed a ~/.irbrc file 

peterv@ASUS:~$ rvm use system
Now using system ruby.
peterv@ASUS:~$ irb --prompt-mode xmp
quit
peterv@ASUS:~$ irb --prompt-mode xmp
001:0> IRB.conf[:PROMPT_MODE]
=> :CUSTOM  ### this was expected to be :XMP