integrate(exp(-x^n),x,0,1) bug for n >2

the definite integral of the real function exp(-x^n) over
the real interval (0,1) is broken (spurious imaginary parts,
wrong real parts) for n = 3 through 15, and probably forever.
-----------------------------------------------------------------
(%i1) exp_test(n) :=
( print (n," qags = ", first( quad_qags (exp(-x^n),x,0,1)),
" integrate = ",
expand(float(rectform(integrate(exp(-x^n),x,0,1))))))$

(%i2) for m thru 15 do exp_test(m)$

1 qags = 0.63212055882856 integrate = 0.63212055882856
2 qags = 0.74682413281243 integrate = 0.74682413281243
3 qags = 0.80751118213967 integrate =
0.69932519757296*%i-0.40375559106984
4 qags = 0.8448385947571 integrate = 0.8448385947571*%i
5 qags = 0.87007466768589 integrate =
0.82749018236601*%i+0.26886785869008
6 qags = 0.88826369875194 integrate =
0.76925892837871*%i+0.44413184937597
7 qags = 0.90199160301324 integrate =
0.70520543215755*%i+0.56238256584096
8 qags = 0.91271857185875 integrate =
0.64538949147622*%i+0.64538949147622
9 qags = 0.9213308364909 integrate =
0.59222004611848*%i+0.70578036756801
10 qags = 0.92839720283799 integrate =
0.54569818409775*%i+0.7510891146261
11 qags = 0.93429939205577 integrate =
0.5051203870693*%i+0.78598266428894
12 qags = 0.93930307080183 integrate =
0.46965153540091*%i+0.81346032116712
13 qags = 0.94359882032526 integrate =
0.43851223691831*%i+0.83551526125626
14 qags = 0.94732690220079 integrate =
0.41102973849353*%i+0.85351204660935
15 qags = 0.95059283193 integrate =
0.38664093739113*%i+0.86840976367727
------------------------------------------------

Maxima version: 5.25.1
Maxima build date: 10:2 9/6/2011
Host type: i686-pc-mingw32
Lisp implementation type: GNU Common Lisp (GCL)
Lisp implementation version: GCL 2.6.8

Ted Woollett