Symmetry check in setindex! for Symmetric/Hermitian (#1317)

Currently, the following works unexpectedly:
```julia
julia> A = Symmetric(fill([1 2; 3 4], 3, 3))
3×3 Symmetric{AbstractMatrix, Matrix{Matrix{Int64}}}:
 [1 2; 2 4]  [1 2; 3 4]  [1 2; 3 4]
 [1 3; 2 4]  [1 2; 2 4]  [1 2; 3 4]
 [1 3; 2 4]  [1 3; 2 4]  [1 2; 2 4]

julia> A[1,1] = [1 2; 3 4]
2×2 Matrix{Int64}:
 1  2
 3  4

julia> A[1,1]
2×2 Symmetric{Int64, Matrix{Int64}}:
 1  2
 2  4
```
After this PR, the `setindex!` throws an error if the exact value cannot
be used:
```julia
julia> A[1,1] = [1 2; 3 4]
ERROR: ArgumentError: cannot set a diagonal element of a symmetric matrix to an asymmetric value
```

Author: jishnub

src/symmetric.jl

@@ -266,6 +266,7 @@ Base._reverse(A::Symmetric, ::Colon) = Symmetric(reverse(A.data), A.uplo == 'U'
 
 @propagate_inbounds function setindex!(A::Symmetric, v, i::Integer, j::Integer)
     i == j || throw(ArgumentError("Cannot set a non-diagonal index in a symmetric matrix"))
+    issymmetric(v) || throw(ArgumentError("cannot set a diagonal element of a symmetric matrix to an asymmetric value"))
     setindex!(A.data, v, i, j)
     return A
 end
@@ -276,8 +277,8 @@ Base._reverse(A::Hermitian, ::Colon) = Hermitian(reverse(A.data), A.uplo == 'U'
 @propagate_inbounds function setindex!(A::Hermitian, v, i::Integer, j::Integer)
     if i != j
         throw(ArgumentError("Cannot set a non-diagonal index in a Hermitian matrix"))
-    elseif !isreal(v)
-        throw(ArgumentError("Cannot set a diagonal entry in a Hermitian matrix to a nonreal value"))
+    elseif !ishermitian(v)
+        throw(ArgumentError("cannot set a diagonal element of a hermitian matrix to a non-hermitian value"))
     else
         setindex!(A.data, v, i, j)
     end

test/symmetric.jl

@@ -1180,4 +1180,15 @@ end
     end
 end
 
+@testset "block-symmetric setindex!" begin
+    A = fill([1 2; 3 4], 2, 2)
+    v = [1 2; 3 4]
+    H = Hermitian(A)
+    h_msg = "cannot set a diagonal element of a hermitian matrix to a non-hermitian value"
+    @test_throws h_msg H[1,1] = v
+    S = Symmetric(A)
+    s_msg = "cannot set a diagonal element of a symmetric matrix to an asymmetric value"
+    @test_throws s_msg S[1,1] = v
+end
+
 end # module TestSymmetric