mutate convert is flattening array of 1 into single element instead of maintaining array

The following creates an array properly (an array of 1 element in some cases):

```
filter {
mutate {
gsub => ["upstream_response_time", "-", null]
split => ["upstream_response_time", ":"]
strip => ["upstream_response_time"]
}
}
```

json output:

```
"upstream_response_time": [
"0.317"
],
```

The following creates flattens that array to just a single value instead of maintaining the array for json output:

```
filter {
mutate {
gsub => ["upstream_response_time", "-", null]
split => ["upstream_response_time", ":"]
strip => ["upstream_response_time"]
+ convert => ["upstream_response_time", "float"]
}
}
```

json output:

```
"upstream_response_time": 0.317,
```

I would like for the array to be maintained and represented in json as such. Is this a bug or feature request? The documents state that all values in the array are casted, not that the array structure is lost.