in Channel put!, convert value to Channel type

`put!(ch::Channel{T}, v)` should convert `v` to type `T`.

This can prevent errors like:

```
julia> c = Channel{Int}(0)
Channel{Int64}(sz_max:0,sz_curr:0)

julia> @async put!(c, :a)
Task (runnable) @0x00007ff5b9d79270

julia> isready(c) && take!(c)
ERROR: TypeError: in take_unbuffered, in typeassert, expected Int64, got Symbol
Stacktrace:
 [1] take_unbuffered(::Channel{Int64}) at ./channels.jl:323
 [2] take!(::Channel{Int64}) at ./channels.jl:306
 [3] top-level scope at none:0
```

base/channels.jl

@@ -246,9 +246,10 @@ Append an item `v` to the channel `c`. Blocks if the channel is full.
 For unbuffered channels, blocks until a [`take!`](@ref) is performed by a different
 task.
 """
-function put!(c::Channel, v)
+function put!(c::Channel{T}, v) where T
     check_channel_state(c)
-    isbuffered(c) ? put_buffered(c,v) : put_unbuffered(c,v)
+    vT = convert(T, v)
+    isbuffered(c) ? put_buffered(c,vT) : put_unbuffered(c,vT)
 end
 
 function put_buffered(c::Channel, v)