Fix hermitian check for `expect`

[ytdHuang]

Description

For the current method, if one calls

O   # Operator
rho # Operator
expect(O, rho)

It will automatically take the real value after tr when depend on whether O is hermitian or not. But the condition of taking the real value should depend on whether O * rho is hermitian or not.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 1.77%. Comparing base (e76d77d) to head (c32c71e).

Files with missing lines	Patch %	Lines
src/qobj/functions.jl	0.00%	2 Missing :warning:

:exclamation: There is a different number of reports uploaded between BASE (e76d77d) and HEAD (c32c71e). Click for more details.

HEAD has 8 uploads less than BASE

| Flag | BASE (e76d77d) | HEAD (c32c71e) | |------|------|------| ||9|1|

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #215 +/- ## ========================================== - Coverage 93.27% 1.77% -91.51% ========================================== Files 29 29 Lines 2172 2085 -87 ========================================== - Hits 2026 37 -1989 - Misses 146 2048 +1902 ```

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