Fix use of @nospecialize with default arguments

[odow]

This is my fault for not reading the docstring carefully. (Although it worked for Julia 1.6.2+, so I guess I never bumped into the problem.)

Here's the docstring, with the relevant sentence:

help?> @nospecialize
  @nospecialize

  Applied to a function argument name, ... When applied to an argument, the
  macro must wrap the entire argument expression, e.g., @nospecialize(x::Real)
  or @nospecialize(i::Integer...) rather than wrapping just the argument name.
  ...

The docstring also contains this example, which I failed to see:

  function example_function(x, @nospecialize(y = 1))
      ...
  end

Closes #3461

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b3c6f9f) 98.00% compared to head (86a3c0c) 98.00%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3464   +/-   ##
=======================================
  Coverage   98.00%   98.00%           
=======================================
  Files          36       36           
  Lines        5069     5069           
=======================================
  Hits         4968     4968           
  Misses        101      101           

Files Changed	Coverage Δ
src/JuMP.jl	94.87% <ø> (ø)
src/optimizer_interface.jl	96.89% <ø> (ø)

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