[with patch, positive review] substitution of a dict into a symbolic expression modifies the dict

[was]

sage: var('v t')
sage: f = v*t
sage: D = {v: 2}
sage: f(D, t=3)
6
sage: D
{v: 2, t: 3}

After the call above, D should *not* be changed. This issue was raised at this  sage-support thread.

[was]

Here's how to workaround it (namely, put "dict(constants)" instead of "constants"). In the code for call or subs, something similar should be done.

sage: var('v t')
sage: f = v*t
sage: s = {v: 2}
sage: f(dict(s), t=3)
6
sage: s
{v: 2}

[wcauchois]

based on sage 4.1.1.alpha1

[wcauchois]

This was a simple fix.