inconsistent parsing behaviour on nested params

[Bert Bruynooghe]

When the query parameters for the request are:
AI.1=d&AI.2=e&AI.3=f&AI.4=g&AI.5=h&AI.6=i&AI.7=j&AI.8=k&AI.9=&AI.10=&AI.11=&AI.12=&AI.13=&AI.14=&AI.15=&AI.16=&AI.17=&AI.18=&AI.19=&AI.20=
request.body returns
{"AI":["d","e","f","g","h","i","j","k","","","","","","","","","","","",""]}

However, with 21 params:
AI.1=e&AI.2=f&AI.3=g&AI.4=h&AI.5=i&AI.6=j&AI.7=k&AI.8=&AI.9=&AI.10=&AI.11=&AI.12=&AI.13=&AI.14=&AI.15=&AI.16=&AI.17=&AI.18=&AI.19=&AI.20=&AI.21=
request.body returns a different structure:
{"AI":{"1":"e","2":"f","3":"g","4":"h","5":"i","6":"j","7":"k","8":"","9":"","10":"","11":"","12":"","13":"","14":"","15":"","16":"","17":"","18":"","19":"","20":"","21":""}}

on version 4.3.0