Fix LinearAlgebra.hermitian(::AbstractJuMPScalar, ::Symbol)

[odow]

Closes #3692

Docstrinng is:

help?> LinearAlgebra.hermitian
  hermitian(A, uplo=:U)

  Construct a hermitian view of A. If A is a matrix, uplo controls whether the upper (if uplo = :U)
  or lower (if uplo = :L) triangle of A is used to implicitly fill the other one. If A is a Number,
  its real part is returned converted back to the input type.

  If a hermitian view of a matrix is to be constructed of which the elements are neither matrices
  nor numbers, an appropriate method of hermitian has to be implemented. In that case,
  hermitian_type has to be implemented, too.

The important sentence is: If A is a Number, its real part is returned converted back to the input type.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.36%. Comparing base (d031be9) to head (8e19007).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3693   +/-   ##
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  Coverage   98.36%   98.36%           
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  Files          43       43           
  Lines        5698     5699    +1     
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+ Hits         5605     5606    +1     
  Misses         93       93           

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