On Windows the .size ShmemConf parameter is not respected

Hello,

On linux if I ask to open a piece of Memory created somewhere else with a size, that size is the size of the buffer returned. On Windows if I do this (for a small buffer) I get 4096 - which I assume is the default smallest size for a piece of Windows shared memory. This means that the producer thinks it has buffer len x, while the consumer thinks it's working with buffer len 4096 (for a small buffer).

I think instead of using `MEMORY_BASIC_INFORMATION.RegionSize` the size from the opts should be taken, do you agree?

I've updated a test [here](https://github.com/3tilley/shared_memory/pull/1) to demonstrate this.
```
$ cargo test
Compiling shared_memory v0.12.5 (C:\Users\Max\Documents\GitHub\shared_memory)
Finished test [unoptimized + debuginfo] target(s) in 0.66s
Running unittests src\lib.rs (target\debug\deps\shared_memory-baddeea1e8dc4909.exe)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Running tests\general.rs (target\debug\deps\general-356fc0d2e34d7138.exe)

running 5 tests
test share_data ... FAILED
test create_new ... ok
test open_os_id ... ok
test create_with_flink ... ok
test open_flink ... ok

failures:

---- share_data stdout ----
thread 'share_data' panicked at tests\general.rs:109:5:
assertion `left == right` failed
left: 4
right: 4096
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:
share_data

test result: FAILED. 4 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s

error: test failed, to rerun pass `--test general`
```

Would you be happy for me to make this change?