Hello,
I did some research and found a wowwiki article back in times.
It's from august 2006.
Just a quote:
Rate at which health points regenerate.
    No regeneration occurs in combat, except for trolls.
    There is 1 tick of regeneration every 2 seconds.
    Health Regeneration is increased while sitting.
    Its value depends on class and spirit:
        Druid:
            Spirit * 0.09 + 6.5 
        Hunter:
            Spirit * 0.25 + ? 
        Mage:
            Spirit * 0.10 + ? 
        Paladin:
            Spirit * 0.25 + ? 
        Priest:
            Spirit * 0.10 + ? 
        Rogue:
            Spirit * 0.50 + 2 Confirmed at level 60 patch 1.09 
        Shaman:
            Spirit * 0.11 + ? 
        Warlock:
            Spirit * 0.07 + 6 
        Warrior:
            Spirit * 0.80 + ? 
(Probably each regen rate should have a constant added.)
    Trolls regenerate 110% of this amount while out of combat, and 10% of this amount while in combat. 
In the revision of february 2007 of this article health regen is increased by 33 % while sitting.
The constant (the ? in my quote) for shamans is 7 and for warriors it is 6.
There is no health regen durin fight through spirt except for trolls. They get just the 10%. 
 - Bilandal